A modulation method based on graphics applied to DAB converter and device

Abstract

The application provides a modulation strategy based on graphing and applied to a DAB converter and device, and belongs to the field of power electronics. The modulation strategy is based on two modes with the highest efficiency that can be realized by the DAB, aiming at the requirements of high power density and low current stress required by the DAB, and a modulation strategy based on graphing analysis and aiming at the optimization of inductance current peak value is provided. The DAB is designed and controlled by a closed-loop PI aiming at the modulation strategy. The optimization goal of minimizing the inductance current peak value of the DAB in the whole power range is effectively realized, the efficiency of the DAB is improved, the current stress of the DAB is reduced, and finally the closed-loop control of the DAB is realized.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a graphical modulation method for optimizing inductor current peak values ​​applied to DAB converters and devices, and a closed-loop PI control system for the proposed modulation method. Background Technology

[0002] With the increasing prominence of global warming and excessive fossil fuel consumption, research on clean energy has received widespread attention from society and academia. Among these, renewable energy, as a component of clean energy, is experiencing significant growth. In recent years, with the continuous development of power electronics technology, DC microgrid systems based on distributed renewable energy generation have become a research hotspot. Dual-Active-Bridge (DAB) DC-DC converters, as energy conversion devices for DC voltage conversion, are widely used in DC microgrids. Therefore, research on DAB modulation methods and efficiency optimization has significant practical value and importance.

[0003] In the modulation methods of dual active full-bridge bidirectional DC-DC converters, the phase shift between each bridge arm is usually used as the optimization quantity for modulation. Currently, commonly used modulation methods in dual active full-bridge bidirectional DC-DC converters include single-phase-shift modulation (SPS), extended-phase-shift modulation (EPS), dual-phase-shift modulation (DPS), and triple-phase-shift modulation (TPS). Among these, TPS, as the most flexible modulation method, is widely used in DAB modulation methods. SPS, DPS, and EPS can all be considered special cases of TPS. However, TPS has three control variables, resulting in up to 12 operating conditions for the DAB converter. The expressions for the DAB's transmitted power and inductor current are different under each condition. Therefore, research on DAB modulation methods faces difficulties in modeling and analysis.

[0004] Meanwhile, as the peak current of the DAB increases, the requirements for the switching transistors and power inductors become more stringent, leading to increased device costs and reduced power density. For example, when the converter operates at a large voltage conversion ratio k, a larger magnetic core is required. Excessive current stress can also reduce the converter's efficiency, and in some cases, even damage the converter's power devices. Therefore, optimizing the peak current of the DAB is one of the important performance indicators in DAB research.

[0005] This paper addresses the challenge of optimizing the three control variables in DAB operation under TPS mode. Two special operating modes are selected, transforming the three control variables into two, thus simplifying the analysis of DAB under TPS operation. Based on this, contour plots of output power and peak current are analyzed for these two operating modes. The optimization of peak inductor current is achieved based on the principle of tangency in the contour plots, and a modulation method is proposed. This modulation method solves the requirements of high power and low current stress for DAB. Furthermore, based on the proposed modulation method, closed-loop PI control is implemented for DAB, ultimately achieving closed-loop control of the DAB. Summary of the Invention

[0006] To address the problems existing in the background technology, the purpose of this invention is to solve the problem of difficulty in optimizing the three control variables in the TPS mode of DAB. Two special operating modes are introduced to transform the three-variable optimization problem into a two-variable optimization problem. Based on this, by drawing the expressions of the output power and the peak inductor current under the two modes, a modulation method is proposed in which the tangent of the power contour line and the slope of the peak inductor current contour line are equal. Finally, a closed-loop PI regulation system is designed based on the proposed modulation method to control its output voltage.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] 1. A graphical optimization modulation method for inductor current peak based on a dual active bridge (DAB) converter, characterized in that:

[0009] 1) By selecting two specific operating modes of the DAB, the power expressions of the DAB under the two different operating modes are obtained;

[0010] The modulation methods for the two selected working modes are shown below:

[0011]

[0012] The expressions for the transmission power of DAB in different modes are:

[0013]

[0014] Where P base =kV1 2 / (8fsL), is the reference power of the normalized power; d1 and d2 are the duty cycles of the primary and secondary bridge arm voltages; d3 is the shift ratio of the rising edge of the primary and secondary bridge arm voltages; k = nV2 / V1 is the defined voltage gain ratio. Here, only the case of k < 1 is analyzed. The analysis of the case of k > 1 is similar to that of the case of k < 1; n is the ratio of the number of turns on the primary side to the number of turns on the secondary side of the transformer; fs is the switching frequency of the DAB converter; L is the power inductance of the DAB converter; V1 is the input voltage on the primary side; and V2 is the output voltage on the secondary side.

[0015] 2) Calculate the peak inductor current under both modes, using the formulas shown below:

[0016]

[0017] Where i base =V1 / (8fsL), which is the reference current for the normalized peak inductor current.

[0018] 3) Based on the above power expression and inductor current peak expression, draw contour plots of transmitted power and inductor current peak under different modes.

[0019] 4) Based on the characteristics of the drawn contour map, an optimized modulation method based on the equality of contour tangents is proposed.

[0020] 5) Design a closed-loop system based on the optimized modulation method proposed in step 4) to control the output voltage.

[0021] 2. The graphical optimization modulation method for inductor current peak value as described in claim 1, characterized in that step 1) specifically comprises:

[0022] Based on the formula for output power and the two selected operating modes, the expression for transmission power is derived, where the formula for calculating transmission power is shown below:

[0023]

[0024] Where T is the switching period of DAB operation, v1(t) is the AC voltage of the primary side bridge arm, and i1(t) is the inductor current of the primary side bridge arm.

[0025] 3. The optimized modulation method based on equal contour tangents according to step 4) of claim 1, specifically, is as follows:

[0026] When k and the transmitted power are constant, there exists a combination of d1 and d3 such that the slope of the tangent line on the contour line is the same as the slope of the contour line of the inductor current. This point is the point where the inductor current stress is minimized under the condition of transmitting a given power. The slope of the tangent line of the power transmission contour line is shown in the following formula:

[0027]

[0028] Where P const This indicates that the power transmitted by DAB is constant at this time.

[0029] The slope of the tangent line to the contour line of the inductor current peak is shown in the following formula:

[0030]

[0031] Where i stress_const This indicates that the peak inductor current of DAB is constant at this time.

[0032] Based on the idea that the slopes of the tangents to contour lines are equal, the slopes of the tangents to the power contour lines under different power conditions are equal to the slope of the tangent to the peak inductor current, that is:

[0033]

[0034] Then, the modulation method that minimizes the peak inductor current under different modes can be derived:

[0035]

[0036] 4. Based on the modulation method finally proposed in claim 3, a traditional PI power control loop is constructed. Substituting the obtained expressions for d2 and d3 into the expression for transmitted power, the relationship between d1, transmitted power, and voltage gain ratio k when transmitting a fixed power is finally obtained, as shown below:

[0037]

[0038] Where p is the per-unit power transmitted. From the above equation, it can be seen that d1 and the transmitted power p are positively correlated under all conditions, that is, d1 increases as p increases. Therefore, d1 can be used as the control parameter of the PI controller to control the transmitted power. The constructed PI control loop is shown below:

[0039]

[0040] Where P control The output of the PI loop directly controls the duty cycle of the primary side bridge arm voltage.

[0041] 5. The present patent proposes an optimized modulation method for the peak value of inductor current based on graphing, which is characterized by including a DAB converter and a control circuit. The DAB converter consists of 8 switching tubes S1 - S4 and S5 - S8 respectively to form the primary and secondary side bridge arms. The DAB converter has three controllable variables, namely the duty cycles d1 and d2 of the voltages of the primary and secondary side bridge arms, where the range is 0 < d1 < 1, 0 < d2 < 1; and the phase shift ratio d3 of the rising edges of the voltages of the primary and secondary side bridge arms, where the range is 0 < d3 < 1. The gate of each switching tube on the primary and secondary sides is connected to the output terminal of the drive circuit, and two switching tubes on the same bridge arm cannot be conducting simultaneously. One end of the inductor L is connected to the midpoint of the primary side bridge arm and one end of the transformer, and the input capacitor C in and the output capacitor C out are respectively connected to the positive and negative poles of the input voltage V1 and the output voltage V2. Description of the Drawings

[0042] Figure 1 is the circuit structure diagram of the DAB converter and its device.

[0043] Figure 2 is the classical working waveform diagram of two selected working modes.

[0044] Figure 3 is the control block diagram of the modulation method provided by the present invention applied to the DAB converter and its device.

[0045] Figure 4 is the 3D diagram and its contour diagram of the transmission power of the DAB converter under light load and heavy load conditions.

[0046] Figure 5 is the 3D diagram and its contour diagram of the peak value of the inductor current of the DAB converter under light load and heavy load conditions.

[0047] Figure 6 is the contour diagram of the overlapping of the transmission power and the peak value of the inductor current of the DAB converter under light load and heavy load conditions.

[0048] Figure 7 is the simulation waveform when the output power is 250W and 30W at k = 0.8.

[0049] Figure 8 is the simulation waveform when the output power is 130W and 30W at k = 0.4.

[0050] Figure 9 is the simulation waveform when V 2_ref changes under PI control. Detailed Implementation Manner

[0051] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings.

[0052] Figure 1 This is the circuit structure diagram of the DAB converter and its device provided by the present invention. The DAB converter consists of eight switching tubes S1 to S8, a power inductor L and a high-frequency transformer HF in the middle of the primary and secondary side bridge arms, an input capacitor C at the input end in and an output capacitor C at the output end out The eight switching tubes S1 to S4 and S5 to S8 respectively form the primary and secondary side bridge arms. One end of the power inductor L is connected to the midpoint of the primary side bridge arm and one end of the transformer. The input capacitor C in and the output capacitor C out are respectively connected to the positive and negative poles of the input voltage V1 and the output voltage V2. The gate of each switching tube is connected to the output end of the drive circuit, and two switching tubes in the same bridge arm cannot be turned on simultaneously.

[0053] Figure 2 This is the classic working waveform diagram of two working modes of DAB selected in the present invention. The DAB converter has three controllable variables, namely the duty ratios d1 and d2 of the voltages of the primary and secondary side bridge arms, with the range 0 < d1 < 1, 0 < d2 < 1; and the phase shift ratio d3 of the rising edges of the voltages of the primary and secondary side bridge arms, with the range 0 < d3 < 1. Among them Figure 2 .1 is the working waveform when the DAB converter works under light load. At this time, the DAB converter works under special triple-phase-shift (TPS) modulation. Since d2 = d1 / k, the control variables of DAB change from three to two, namely d1 and d3, which simplifies the difficulty of analyzing the working mode of DAB. And Figure 2 .2 is the working waveform when the DAB converter works under heavy load. At this time, the DAB converter works under extended-phase-shift (EPS) modulation. At this time, d2 = 1, and the control variables of DAB are also d1 and d3, which also simplifies the analysis difficulty of the DAB converter.

[0054] Figure 3 This is the control block diagram of the proposed modulation method applied to the DAB converter and its device in the present invention. The control block diagram consists of the DAB main circuit and the digital implementation flowchart. Among them, the sampling circuit samples the input voltage V1 and the output voltage V2 and sends them into the digital controller. At the same time, the reference voltage V 2_ref is also sent into the reference voltage. The digital controller judges the current working mode of the DAB converter through the input V 2_ref and V2, and selects d 2, d3 in the current mode and Pcontrol The relationship between them is fed into the next level to prepare for the calculation of d1 and d2. 2, d3; at the same time, the digital controller will V 2_ref The signal from V2 is fed into the PI controller, and the PI controller ultimately outputs P. control Used to calculate d1,d 2, d3. Based on d1, d 2, d3 generates a PWM square wave signal and sends it to the drive circuit. Finally, the drive circuit drives the switching transistors S1~S8 to control the DAB converter.

[0055] Figure 4 shows the 3D and contour images of the transmitted power in two different modes when k=0.8. Figure 4.1 .1 and Figure 4.1 .2 are 3D images and contour images showing the power that DAB can transmit under light load, respectively; and Figure 4.1 .1 and Figure 4.2 Figure 2 shows the 3D image and contour image of the power that the DAB can transmit under heavy load, respectively. From the 3D image and contour image, it can be seen that when the power transmitted by the DAB is constant, the corresponding d of the fixed power that can be transmitted is... 1, There are countless combinations of d3, and its contour lines exhibit a parabolic shape.

[0056] Figure 5 shows the 3D and contour images of the peak inductor current in two different modes when k=0.8. Figure 5.1 .1 and Figure 5.1 .2 are 3D images and contour images of the peak value of the DAB inductor current under light load, respectively; while Figure 5.2 .1 and Figure 5.2 Figure 2 shows the 3D image and contour image of the peak value of the DAB inductor current under heavy load. From the 3D image and contour image, it can be seen that the contour lines of the peak value of the DAB inductor current are a straight line, and the slope of each contour line is the same. However, the slope of the contour lines gradually increases from the lower left to the upper right.

[0057] Figure 6 The graph shows the contour plots of the DAB converter's transmission power and peak inductor current under light and heavy load conditions with k=0.8. Figure 6 .1 is a contour plot of the power transmitted by the DAB converter and the peak inductor current under light load. Figure 6Figure 2 shows the contour plot of the power transmitted by the DAB converter and the peak inductor current under heavy load. Under light load, it can be seen that when the transmitted power is constant, the peak inductor current of the DAB decreases as d2 decreases. When d2 equals 0, the required power can be transmitted while minimizing the peak inductor current. This also means that when the slope of the tangent of the power contour line is equal to the slope of the inductor current contour line, the contour lines intersect at only one point, and the peak inductor current of the DAB is minimized. Under heavy load, when the DAB transmits a certain amount of power, the tangent value of the corresponding inductor current contour line is also constant, and the power contour line exhibits a parabolic shape. The peak inductor current gradually decreases from the upper left to the lower right. Therefore, when the slope of the tangent of the power contour line is equal to the tangent of the peak inductor current, that is, when the two contour lines intersect at only one point, the minimum peak inductor current control can be achieved.

[0058] Table 1

[0059]

[0060] Figure 7 Simulation waveforms for output power of 30W and 130W when k=0.4 are shown. The simulation parameters are shown in Table 1, and the remaining simulation parameters are the same as in Table 1, and will not be repeated hereafter. Among them, from... Figure 7 As can be seen from .1,

[0061] Figure 7 .1 represents an important waveform when k=0.4 and the output power is 30W, while Figure 7 The waveform is shown when k=0.4 and the output power is 130W.

[0062] Figure 8 Simulated waveforms with output power of 30W and 250W when k=0.8. Figure 8 .1 represents an important waveform when k=0.8 and the output power is 30W. Figure 8 The waveform when k=0.8 and the output power is 250W.

[0063] Figure 9 To output a reference value V under PI control 2_ref The dynamic waveform during change. (Among them) Figure 9 .1 is V 2_ref The dynamic waveform when changing from 20V to 50V, and Figure 9 .2 is V 2_ref The dynamic waveform when the voltage changes from 50V to 20V.

[0064] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A graphical modulation method for DAB converters and devices, characterized in that: Step 1: By selecting two specific operating modes of the DAB, the power expressions of the DAB under the two selected different operating modes are obtained; The modulation methods for the two selected working modes are shown below: Where p represents the power transmission rate or normalized power; the expressions for the power transmitted by the DAB in different modes are: in: P base =kV 1 2 / ( 8f s L ), which is the reference power after normalization; d 1 , d 2 represents the duty cycle of the primary and secondary bridge arm voltages; d 3 is the shift of the rising edge of the primary and secondary bridge arm voltages; k=nV 2 / V 1 , is the defined voltage gain ratio; n This represents the ratio of the number of turns on the primary side to the number of turns on the secondary side of the transformer. f s The switching frequency of the DAB converter. L For the power inductor of the DAB converter, V 1 This is the input voltage on the primary side. V 2 This is the output voltage on the secondary side; Step 2: Calculate the peak inductor current under both modes, using the formulas shown below: in, i base =V 1 / (8 f s L ), which is the reference current for the normalized peak value of the inductor current; Step 3: Based on the above power expression and inductor current peak expression, draw contour plots of transmitted power and inductor current peak under different modes. Step 4: For the drawn contour map, propose an optimized modulation method based on the equality of contour tangents; Step 5: Design a closed-loop system based on the optimized modulation method proposed in Step 4 to control the output voltage.

2. The modulation method based on graphical representation applied to DAB converters and devices as described in claim 1, characterized in that, The specific content of step 1 is as follows: Based on the formula for output power and the two selected operating modes, the expression for transmission power is derived, where the formula for calculating transmission power is shown below: in: T The switching cycle for DAB operation, v 1 ( t ) represents the AC voltage of the primary side bridge arm. i 1 ( t ) represents the inductance current of the primary side bridge arm.

3. The modulation method based on graphical representation applied to DAB converters and devices as described in claim 1, wherein the optimized modulation method based on equal contour tangents in step 4 is specifically based on the following idea: When the voltage gain ratio k is constant and the transmitted power is constant, there will be a certain... d 1 and d The combination of points 3 and 4 results in a point where the slope of the tangent line on the contour line is the same as the slope of the contour line for the inductor current. This point is the point where the inductor current stress is minimized under the condition of transmitting a given power. The slope of the tangent line on the contour line for transmitting power is shown in the following formula: in, P const This indicates that the power transmitted by DAB is constant at this time; The slope of the tangent line to the contour line of the inductor current peak is shown in the following formula: in: i stress_const This indicates that the peak inductor current of DAB is constant at this time; Based on the idea that the slopes of the tangents to contour lines are equal, the slopes of the tangents to the power contour lines under different power conditions are equal to the slope of the tangent to the peak inductor current, that is: in: i stress_const If defined as a constant value, then the modulation method that minimizes the peak value of the inductor current under different modes can be obtained; 。 4. The graphical modulation method for DAB converters and devices as described in claim 1, constructing a traditional PI power control loop; and obtaining... d 2, d Substituting the expression for 3 into the expression for transmission power, we finally arrive at the result when the transmission power is fixed. d 1. Transmission power and voltage gain ratio k The relation is as follows: in: p The per-unit power transmitted; From the above formula, we can see that d 1. Power of transmission p A positive correlation is shown, that is d 1 With p It increases with the increase of; therefore, it can be utilized d 1 is used as the control parameter of the PI controller to control the transmitted power; the constructed PI control loop is shown below: in: P control The output of the PI loop directly controls the duty cycle of the primary side bridge arm voltage.

5. A graphical modulation method for DAB converters and devices as described in claim 1, characterized in that, It includes a DAB converter and control circuitry; the DAB converter consists of 8 switching transistors. S 1~ S 4. S 5~ S The eight components form the primary and secondary bridge arms. The DAB converter has three controllable variables: the duty cycle of the voltage in the primary and secondary bridge arms. d 1 and d 2, its range is 0 < d 1<1、0< d 2 < 1; as well as The shift of the rising edge of the primary and secondary bridge arm voltages d 3, its range is 0 < d 3<1; The gate of each switching transistor on the primary and secondary sides is connected to the output terminal of the drive circuit, and the two switching transistors on the same bridge arm cannot be turned on simultaneously. Inductance L One end is connected to the midpoint of the primary side bridge arm and one end of the transformer, and the input capacitor... C in With output capacitor C out respectively with input voltage V 1 and output voltage V The positive and negative terminals of 2 are connected together.

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